Method of producing ductile coated steel and novel product



United States Patent I 3,355,265 METHOD OF PRODUCING DUCTILE COATEDSTEEL AND NOVEL PRODUCT Robert M. Hudson, Churchill Borough, AlleghenyCounty, and Andrew Lesney, Frazer Township, Allegheny County, Pa.,assignors to United States Steel Corporation, a corporation of DelawareNo Drawing. Filed Apr. 16, 1965, Ser. No. 448,875 14 Claims. (Cl.29-1835) ABSTRACT OF THE DISCLOSURE A method of producing ductile coatedsteel product having a coating of, for example, chromium, copper, nickeland titanium, which comprises cold reducing a metallurgically bondedcoated steel strip and annealing the coated steel strip at a temperaturebelow the Ac; temperature to completely recrystallize the strip withoutcausing substantial alloying of the steel and coating metal. Alsodisclosed herein is a fully annealed coated steel foil 0.004-inch thickor less having a protective coating of, for example, one of theaforementioned coating metals, metallurgically bonded thereto but notcompletely alloyed therewith.

This invention relates to a method for producing ductile coated steelproduct and to a novel product. More particularly, the invention isdirected to a method of producing steel having a protective metalcoating and which is soft and has excellent folding endurance. Themethod is pain ticularly suitable to the production of ductile coatedsteel in very light gauges and, in particular, ductile coated steelfoil.

There is currently a need for steel product in light gauges whichcontains a coating of protective metal. Uncoated steel which isavailable with good ductility possesses unsatisfactory properties formany purposes with respect to corrosion resistance. To improve itssurface properties, steel has been coated with a variety of materials,the more common being tin, zinc and lead. The recent innovation of steelfoil, i.e. steel which is cold reduced to a thickness of less than 0.004inch, has held out the promise of a significantly improved packagingmaterial replacing such materials as aluminum, plastics and paper.Plastic and paper do not provide adequate resistance to penetration ofvapors and moisture and, along with aluminum, have a low tearingstrength and puncture resistance. In contrast, the high strength andexcellent chemical, biological and radiological resistance of steel foilrenders it well suited for use as a packaging material. Coated steelfoil would add improved surface properties to the already impressivelist of steel foil advantages.

In conventional manufacture of coated steel products such as tin plate,galvanized sheet and plate, etc., uncoated steel, referred to as blackplate is cold reduced on a cold-reduction mill generally before, butsometimes after coating. The cold reduction increases the strength ofthe steel but also increases the steels hardness and stiffness, which ofcourse, means that the steel has a low' ductility. Cold reducing steelto foil gauge, i.e. less than 0.004 inch, greatly lowers ductility andthefoil may be unsuitable for many uses, particularly in the packagingfield where fonmability is very important. Thus, the poor ductilityseverely limits the use of the coated steel product by restricting itsability to be shaped or fabricated and the full potential of coatedsteel in light gauges carinot be realized.

Ordinarily, cold-reduced steel can be softened by annealing; however,coated steel products cannot be conventionally annealed without unduereduction in strength and loss of the protection olfered by the coatingbecause of alloying between the steel base and the coating. Thesedifiicul-ties are particularly troublesome where light-gauge steel suchas foil is involved. Coated steel foil will usually have a very thinmetal coating, and such a thin layer may readily alloy with the steelbase or, in some instances, may react with carbon in the steel to formcarbides of a generally brittle nature which make shaping withoutcracking virtually impossible. The present invention provides a methodfor producing coated steel in a ductile condition which has excellentfolding endurance and, therefore, in light gauges is very suitable forpackaging applications.

According to the present invention, a coated-steel product is coldreduced to desired thickness and then annealed without causingsubstantial alloying of the steel and coating. The annealing isperformed at a temperature below the critical (A0 temperature to avoidtransformation of the steel microstructure and complete alloying withthe coating. The A0 temperature is the temperature at which on heating,the body-centered cubic structure, ferrite, changes to face-centeredcubic, austenite. For steels having above 0.0Q5 carbon, this temperatureis 1333 F. Metal coatings useful in our method are those which have amelting point sufiiciently high to permit complete recrystallization ofthe steel during annealing without completely alloying with the steel.Such metals suited for use with the invention include chromium, copper,nickel and titanium which have melting points that permit annealing ofthe steel without complete alloying with it. I

Although some alloying is desirable to insure a chemical bond, i.e.metallurigical bond, between the coating and the steel base, substantialalloying of the steel and the coating should be avoided in order to:maintain the integrity of the coating and the improved properties andcorrosion resistance which it provides. A metallurgical bond is muchstronger than a mere mechanical bond and is less likely to peel or flakeotf during cold reduction; however, as pointed out above, completealloying of the coating and base is to be avoided to preserve theintegrity of the coating and its value as a protective surface for thesteel. If the coating completely alloys with the steel, the resultingalloy will not possess the protective characteristics of the unalloyedmetal coating. The invention provides a method for making a coated steelproduct in which the integrity of the coating is maintained but whichresults in a completely recrystallized steel base which is ductile andpossesses a high folding endurance.

Some metals useful as protective coatings for steel undergo chemicalreaction with uncombined carbon in steels to form carbides. The carbidesare generally very brittle and when present in sufiicient quantity makeit impossible to satisfactorily shape the steel. Furthermore, thecarbides do not otter the same surface appearance or degree ofprotection to the steel base. When coating materials are used whichreact with uncombined carbon, it is necessary to employ either steelswhich have been decarburized or carbon stabilized steels. It has beenfound that in the case of plain carbon, decarburized steels, the carboncontent should be brought to a level below 0.007 percent (by weight) toprovide a satisfactory steel base without undue carbon reaction with thecoating. Decarburization is, however, fairly time consuming and for thisreason carbon stabilized steels may be preferred. The term carbonstabilized steels as used herein refers to steels in which a knowncarbon stabilizer such as titanium, vanadium, etc. has been added whichlowers the level of uncombined carbon to 0.007 percent or less. In suchsteels, the stabilizer is present according to known relationships insufiicient quantity so that the carbon content is at the desired level.Thus, for example, titanium may be used at a ratio of four partstitanium to one part carbon to lower the uncombined carbon content.However, not all potentially useful coating metals undergo reaction withcarbon to form undesirable carbides. Some metals, such as nickel, may beused without danger of brittle carbide formation and would not requirethe aforementioned carbon restrictions.

Although, as pointed out above, a number of metal coatings are suitablein practicing the invention, chromium coated products are currently ofparticular interest to industry, and accordingly, specific examplesherein will be directed to this embodiment. In the preferred practice ofthe invention, decarburized steels are used. In decarburizing accordingto the preferred practice, plain carbon cold rolled steel isdecarburized as an opened coil in a decarburizing and reducingatmosphere typically containing hydrogen, water vapor and inert gas. Thedecarburization is generally conducted at a temperature in the range of1250 F. to 1350 F. for a time sufficient to lower the carbon content tothe desired level. Decarburization can be performed in any suitablereducing atmosphere. Where hydrogen and water vapor are used, as in thepreferred embodiment, the hydrogen need not be as pure as is necessaryin annealing chromized steel to preserve a characteristic brightappearance. The metal coating, e.g. chromium, may be advantageouslyapplied by circulating chromium-halogen-containing gases through thespaces between the wraps of the open coil according to known chromizingtechniques. After coating, e.g. chromizing, the coated steel coil can becold rolled to foil gauge. The coated steel foil is then annealed,preferably in a high-purity hydrogen atmosphere, either by boxannealing, i.e. batch annealing, or continuous annealing to completelyrecrystallize the steel but without causing substantial alloying of thesteel and coating.

Because of the short time periods involved, continuous annealing is lessarduous and the conditions employed are less critical. For example, thehydrogen atmosphere need not have as high a degree of purity as thatrequired for box annealing where much longer time is involved.Conventional continuous annealing lines may be used and the upperportion of the 1050 F. to 1300 F. range is more useful with continuousannealing than the batch annealing. Satisfactory product has beenproduced by continuous annealing coated steel foil at l200' F. to 1300F. for up to 40 seconds; the preferred conditions being 125 F. for 20 to30 seconds. Because of difficulties associated with handling foil gaugesin a continuous annealing operation (e.g. strip breakage) box annealingoffers some advantages over the continuous treatment.

For a particularly outstanding surface appearance of chromium coatedfoil, it is necessary to employ controlled annealing conditions and anatmosphere containing very high-purity hydrogen. It has been found thatthe atmosphere should contain hydrogen of a purity attainable bydiffusion through a heated (450 C.) palladium-silver alloy membrane.Prior to heating, the system should be carefully purged to insure thatthe gas purity in the annealing chamber will be adequate at the elevatedtemperatures. It has been further found that initial reduction of thegas pressure to about 5 mm. is helpful. During heating, occludedoxidizing gases such as water vapor and oxygen will evolve from the coiland it is advantageous to preheat the coil to about 600 F. to 900 F. andto maintain this temperature for at least four hours before furtherheating. The chromized steel foil should be wrapped on a stainless steelrather than a carbon steel sleeve to minimize the quantity of evolvedimpurity gases from this source. The high-purity hydrogen gasfiow rateused during box annealing should be as high as practicable and ifpossible to a maximum of 200 cubic feet per hour for a single coil-standbase. After this preheating stage, the coil is then heated to anannealing temperature for recrystallization in the range of about 1050F. to about 1300" F. for a time sufficient to completely recrystallizethe steel, but insufiicient to cause any substantial alloying of thecoating and the steel. The preferred temperature for box annealing is inthe range of l050 F. to 1150 F. Rapid cooling to a temperature at whichno gas-metal reactions occur, e.g. about 400 F. is desirable after whichhydrogen may be purged from the system with nitrogen or other suitablegas prior to uncovering the coil.

As a typical example of the preferred embodiment of the invention, acoil of plain-carbon cold rolled steel of about 0.0359 inch thick andwith a carbon content of 0.10 percent was decarburized by open-coilannealing in an atmosphere of about 9 percent hydrogen and the balancenitrogen at a temperature of about 1275 F. In this operation, the carboncontent was reduced to about 0.006 percent. The opened coil was thencoated with chromium by vapor deposition to provide a one mil (0.001inch) thick coating. The coated steel was then cold reduced about 97percent to a foil gauge of approximately 0.0010 inch. The foil was thencleaned according to the preferred practice to remove the rollinglubricant which was used. A portion of the foil was continuous annealedat a temperature of 1250 F.; another portion was annealed in coil formusing the aforementioned box annealing techniques. Upon micrographicexamination, both specimens were shown to possess a smooth substantiallyuniform chromium coating metallurgically bonded to the steel base and auniform surface without jagged raised portions indicative of cracking orflaking.

Table I below shows the properties of coated steel foil producedaccording to the procedure described herein. For contrast, theproperties of the coated foil are compared with aluminum foilproperties.

AzChromized steel foil continuous annealed.

Bzcliromized steel foil box annealed.

C=Alun1inum foil box annealed.

It is apparent that various changes and modifications may be madewithout departing from the invention. For example, various coatingtechniques may be employed to apply a protective metal covering to asteel base. For drastic cold reduction, as is performed in theproduction of steel foil, the coating should be metallurgically bondedto the steel base to minimize or prevent peeling, cracking or flaking ofthe coating during cold reduction. A preferred annealing temperaturerange of 1050 F. to 1300 F. has been presented herein; however, itshould be understood that lower or slightly higher temperatures may beemployed as long as the cold reduced steel is completely recrystallizedto lower its hardness and increase its ductility and complete alloyingbetween the coating and steel base is avoided.

We claim:

1. A method for producing a ductile coated steel foil comprising coldreducing steel having a metallurgically bonded coating of a metal fromthe group consisting of chromium, copper, nickel, and titanium andannealing said cold reduced, coated steel at a temperature below the (Actemperature for a time suflicient to completely recrystallize said steelbut insuflicient to cause substantial alloying of said coating and saidsteel at the coating metal surface.

2. A method of producing ductile coated steel foil comprising applying aprotective metal coating from the group consisting of chromium, copper,nickel and titanium to a steel from the group consisting of plain carbonsteel decarburized to a carbon level of less than about 0.007 percentand carbon stabilized steel having an uncombined carbon content of notgreater than 0.007%, cold rolling said coated steel to foil gauge andannealing said cold rolled, coated steel at a temperature below the (Actemperature for a time suflicient to completely recrystallize said steelbut insufiicient to cause substantial alloying of said coating and saidsteel at th coating metal surface.

3. A method according to claim 2 wherein said coated steel foil isannealed at a temperature in the range of about 1050 F. to about 1300 F.

4. A method of producing ductile steel foil having a protective metalcoating comprising applying a coating of chromium to steel strip, saidsteel being selected from the group consisting of plain carbon steeldecarburized to a carbon level of less than about 0.007 percent andcarbon stabilized steel, cold rolling said coated steel strip to foilgauge and annealing said coated steel foil at a temperature in the rangeof about 1050 F. to about 1300 F. for a time sufiicient to completelyrecrystallize said steel but insuflicient to cause substantial alloyingof said steel and said chromium metal coating at the surface of thechromium coating.

5. A method according to claim 4 wherein said coated steel foil is boxannealed at a temperature of about 1050 F. to about 1200 F., for up toabout five hours.

6. A method according to claim 5 wherein said box annealing is conductedin an atmosphere containing highpurity hydrogen.

7. A method according to claim 4 wherein said coated steel foil iscontinuously annealed at a temperature of about 1200 F. to about 1300 F.for up to about 40 seconds.

8. A method for producing ductile steel foil having a protective metalcoating comprising decarburizing an open coil of plain carbon steel in areducing atmosphere to a carbon level of less than about 0.007 percent,vapor coating a protective layer of chromium onto said decarburizedsteel, cold rolling said chromium coated steel to foil gauge andsubsequently annealing said coated steel foil in an atmospherecontaining high-purity hydrogen and inert gas at a temperature in therange of about 1050 F. to about 1300 F. for a time sufiicient tocompletely recrystallize said steel but insufficient to causesubstantial alloying of said steel and said chromium coating at thesurface of the chromium coating.

9. A method according to claim 8 wherein said steel is decarburized inan atmosphere containing water vapor, hydrogen and inert gas.

10. A method for producing ductile steel foil having a protective metalcoating comprising heating said foil to decarburizing temperature in areducing atmosphere to decarburize said steel to a carbon level of lessthan about 0.007 percent, coating a protective layer of chromium ontosaid decarburized steel, cold rolling said chromium coated steel to foilgauge, heating the cold rolled coated steel foil to about 600 F. to 900F. in a high-purity hydrogen atmosphere to evolve oxidizing gases fromthe coil, subsequently annealing said coated steel foil in saidhigh-purity hydrogen atmosphere at a temperature in the range of about1050 F. to about 1300 F. for a time suflicient to completelyrecrystallize said steel but insufiicient to cause substantial alloyingof said steel and said chromium coating at the surface of the chromiumcoating.

11. A method according to claim 10 wherein, after annealing, the steelis rapidly cooled to a temperature below which gas-metal reactions willnot occur and then the reducing atmosphere is purged from the systemwith inert gas.

12. A method for producing ductile steel foil having a protective metalcoating comprising heating said foil to decarburizing temperature in areducing atmosphere to decarburize said steel to a carbon level of lessthan 0.007 percent, vapor coating a protective layer of chromium ontosaid decarburized steel, cold rolling said chromium coated steel to foilgauge, heating the cold rolled coated steel foil at about 600 F. to 900F. in a high-purity hydrogen atmosphere to evolve oxidizing gases fromthe coil, subsequently annealing said coated steel foil in saidhigh-purity hydrogen atmosphere at a temperature in the range of about1050 F. to about 1150 F. for a time suflicient to completelyrecrystallize said steel but insuflicient to cause substantial alloyingof said steel and said chromium coating at the surface of the chromiumcoating and cooling said annealed coil in said high-purity hydrogenatmosphere to a temperature below about 400 F. before withdrawing saidcoil from said high-purity hydrogen atmosphere.

13. A steel foil product comprising a fully annealed, cold rolled steelbase less than 0.004 inch thick having a protective metal coating fromthe group consisting of chromium, copper, nickel and titaniummetallurgically bonded thereto at the interface but not completelyalloyed therewith at the surface of said coating.

14. A steel foil product according to claim 13 wherein said steel has aprotective coating of chromium.

References Cited UNITED STATES PATENTS 3,058,856 10/1962 Miller 148-163,095,361 6/1963 Stone 20429 3,123,493 3/1964 Brick 117-50 3,214,82011/1965 Smith et al. 2918 3,244,565 4/1966 Mayer et al. 148-42 3,281,26210/1966 Brick 117-50 3,285,790 11/1966 Lockwood 14812.1

DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner.

H. F. SAITO, Assistant Examiner.

13. A STEEL FOIL PRODUCT COMPRISING A FULLY ANNEALED, COLD ROLLED STEELBASE LESS THAN 0.004 INCH THICK HAVING A PROTECTIVE METAL COATING FROMTHE GROUP CONSISTING OF CHROMIUM, COPPER, NICKEL ANDTITANIUMMETALLURGICALLY BONDED THERETO AT THE INTERFACE BUT NOTCOMPLETELY ALLOYED THEREWITH AT THE SURFACE OF SAID COATING.